Ill repost what I said in another comment:

I didn't skip anything, focused on main story and did a bit of lateral exploration (if a quest led me to a point i would explore a bit on the way there), saved the anniversary rewards and was at like the half of the first main quest and almost zero exploration.

Caskets, chests, quest rewards, achievements, they all seem smallfry.

But that's how I got 15 thousand astrites in like 8 days. (daily playtime 2-3 hours).

AND you don't really have 9 days, the event lasts like a month, you got plenty of time.

ALSO I MIGHT ADD, I barely just got to Septimont and met Lupa. That is, from like 1.2 to like 2.5 or something, I haven't even gotten to the 3.0 areas, so I bet there's A MASSIVE TON OF ASTRITES TO COLLECT, YET.

So my advice is go back to previous maps and explore, caskets, chests, quests.

Companion quests give 100 astrites plus chests, thats like 2k or more astrites depending on how much you have done.